(=-96 deg.

F.) by Dulong and Petit's law; but adds, that a closer determination has been made by Professor Moulton, using Stefan's law, that radiation is as the _/4th_ power of the temperature, whence results a mean temperature of-31 deg.

F.These estimates assume identity of atmospheric conditions of Mars and the Earth.
But as none of these estimates take account of the many complex factors which interfere with such direct and simple calculations, Mr.Lowell then proceeds to enunciate them, and work out mathematically the effects they produce: (1) The whole radiant energy of the sun on striking a planet becomes divided as follows: Part is reflected back into space, part absorbed by the atmosphere, part transmitted to the surface of the planet.

This surface again reflects a portion and only the balance left goes to warm the planet.
(2) To solve this complex problem we are helped by the _albedoes_ or intrinsic brilliancy of the planets, which depend on the proportion of the visible rays which are reflected and which determines the comparative brightness of their respective surfaces.

We also have to find the ratio of the invisible to the visible rays and the heating power of each.
(3) He then refers to the actinometer and pyroheliometer, instruments for measuring the actual heat derived from the sun, and also to the Bolometer, an instrument invented by Professor Langley for measuring the invisible heat rays, which he has proved to extend to more than three times the length of the whole heat-spectrum as previously known, and has also shown that the invisible rays contribute 68 per cent, of the sun's total energy.[9] [Footnote 9: For a short account of this remarkable instrument, see my _Wonderful Century_, new ed., pp.